This invention relates to a method of producing aluminum can sheet having high strength and low earing characteristics and more specifically to hot rolling aluminum can sheet ingot in a single-stand reversing hot finish mill and subsequently cold rolling using an intermediate gauge continuous anneal before cold rolling the sheet again to its final thickness.
In the formation of aluminum can sheet into cup-shaped articles, a characteristic known as "earing" manifests itself as a scalloped appearance around the top edge of the formed cup. The scallops, or ears as they are more commonly known, are formed during the deep drawing step in the fabrication of the cup and represent an undesirable feature of the article. In can body stock the cup is subsequently ironed in multiple rings which accentuates the scalloped ears. High earing, therefore, can create transport problems with the cup as well as insufficient trim after ironing, clipped ears, and trimmer jams all of which are unacceptable in can manufacturing. Thus, it is desired to minimize earing in aluminum can sheet in order to avoid these costs and to increase the quality of the cup. See U.S. Pat. No. 3,318,738, the disclosure of which is incorporated herein by reference.
It is also desirable to provide high strength in aluminum can sheet. Strength is measured by evaluating the yield strength after subjecting tensile specimens to an air temperature of 204.degree. C. (400.degree. F.) for 20 minutes. This process simulates the lacquer curing process of the formed and coated can and is therefore used as an indication of the finished can strength. As is well known, the yield strength measurement is obtained from a tension test in which a specimen is subjected to increasing axial load until it fractures. The yield strength is defined as the stress which will produce a small amount of permanent deformation.
Heretofore, in order to obtain acceptable low earing and high strength characteristics, a multiple-stand hot continuous mill was usually employed. As can be seen in FIG. 1, this involves providing one or more hot reversing or breakdown mills 10 which roll the 12 inch-24 inch thick incoming ingot 12 to an intermediate gauge slab 14. The intermediate gauge slab 14 is then introduced into a series of 3-6 (three are shown in FIG. 1) hot rolling stands 16, collectively referred to as a "hot continuous mill", to reduce the intermediate gauge slab 14 to an approximately 0.1 inch thick final hot rolled sheet 17. The final thickness hot rolled sheet 17 is then coiled on a coil 18. The coil 18 can then optionally be batch annealed at 315.degree. C. to 426.degree. C. (600.degree.-800.degree. F.) for 1-6 hours in a furnace 20. After this the sheet 17 is unwound from coil 18 and cold rolled in a cold rolling mill 22 to produce the final thickness sheet 24 (0.0100-0.0130 inches thick) which is subsequently wound onto coil 26.
The hot continuous mill, while effective in producing low earing and high strength characteristics in the aluminum can sheet, represents a major capital expenditure. In addition, this process requires extensive coordination of the individual roll stands within the continuous mill for successful fabrication. Thus, it would be desirable to eliminate the hot continuous mill in order to substantially reduce the capital expense for producing aluminum can sheet, but only if aluminum can sheet can be produced which has low eating and high strength characteristics.
However, until the invention described herein, attempts to produce aluminum can sheet by eliminating the hot continuous mill have not been successful in producing aluminum can sheet having commercially acceptable low earing and high strength characteristics. These known processes in which the hot continuous mill has been eliminated produce aluminum can sheet having either unacceptably high earing or unacceptably low post-bake yield strength or both.
Thus, there is a need for an improved method of producing aluminum can sheet in a single-stand hot reversing mill that has low earing and high strength characteristics.